1. Field of the Invention
This invention relates to a solid polymer electrolyte membrane of a fuel cell and a method for producing the solid polymer electrolyte membrane.
2. Description of the Background
To cope with environmental and resources issues, for example carbon monoxide (CO2) emissions, exhaustion of oil resources and so on, solid electrolyte membrane fuel cells have been widely examined. Solid electrolyte membrane fuel cells have high energy density and do not need long charging times. Thus, solid electrode membrane fuel cells have been rapidly developed all over the world.
Generally, solid polymer electrolyte membrane fuel cells include a proton-conductive solid polymer electrolyte membrane. The solid polymer electrolyte membrane fuel cells are apparatuses for generating electromotive forces or inducing electric currents by electrochemical reactions between fuel gas (for example H2 gas) and oxidizer gas.
A solid polymer electrolyte membrane fuel cell produces the following electrochemical reaction using H2 gas as the fuel gas and using O2 gas as oxidizer gas. The solid polymer electrolyte membrane fuel cell produces the next reaction at an anode electrode side.2H2-->4H++4e−  (1) 
After the electrochemical reaction, the resulting proton (H+) passes through the solid polymer electrolyte membrane within the solid polymer electrolyte membrane fuel cell. Then the solid polymer electrolyte membrane fuel cell produces the next reaction at a cathode electrode.4H++O2+4e−-->2H2O  (2) 
Accordingly, the electrolyte fuel cell produces the electromotive force between the anode and cathode electrodes.
In practical application of the fuel cells, several problems remain to be overcome. Conventional electrolyte membranes are mainly formed by perfluoro carbon sulfonic acid resin. Since electrolyte membranes made of perfluoro carbon sulfonic acid resin are expensive, the cost of these electrolyte membranes is high.
To reduce manufacturing costs, various solid electrolyte membranes employing polyethylene, ethylene tetrafluoroethylene ETFE and so on have been devised. The above solid electrolyte membranes are polymerized with anion groups, for example, sulfonic acid groups, by means of radiation graft polymerization. For example, a conventional solid electrolyte membrane for a fuel cell (ion exchanging membrane) is disclosed in Japanese Patent Application Publication published on Feb. 21, 1995 as Toku-Kai-Hei 07-050170. The solid electrolyte membrane is a polyolefin resin membrane polymerized with sulfonic acid groups. Another conventional solid electrolyte membrane for a fuel cell is disclosed in Japanese Patent Application Publication published on Apr. 15, 1997 as Toku-Kai-Hei 09-102322. This solid electrolyte membrane for the fuel cell is made of a main chain structure formed by a copolymer wherein perfluoro vinyl monomers are polymerized with hydrocarbon vinyl monomers and a hydrocarbon sub chain structure including sulfonic acid groups.
The above conventional solid electrolyte membranes have several problems as follows. Through the radiation graft polymerization, the main chains are cut to be shortened by radiation beams, and crystallization degrees of the substrates are decreased because of grafting in the main chain of the polymers orderly arranged in the polymer substrate. Furthermore, cross-linking between the side chains in the graft polymerization hardens the grafted portion. The substrates drastically decrease the mechanical strength. On the other hand, the substrates include more sulfonic acid groups as hydrophilic groups, thus the percentage of water content of the solid electrolyte membranes is increased. Because of the above moisture content, the solid electrolyte membranes can be widely changed in dimension, thus the solid electrolyte membranes may be broken.